1. Technical Field
The present invention relates to a fuel supply system which supplies fuel to an internal combustion engine.
2. Related Art
In recent years, an improvement in fuel efficiency of an internal combustion engine (hereafter called an engine) has been requested because of environmental issues, but as a fuel supply system which supplies fuel to the engine, among heretofore known apparatus, there is a fuel supply system arranged in such a way as to inject fuel into an inlet port of an engine (for example, refer to Patent Document 1), and a fuel supply system arranged in such a way as to directly inject fuel into a cylinder of an engine (for example, refer to Patent Documents 2 and 3).
The heretofore known apparatus shown in Patent Document 1 is configured of a motor-driven fuel pump (for example, a Wesco type fuel pump) in a fuel tank, a pressure adjustment device, and a fuel injection valve, and fuel from the fuel pump is adjusted to a constant pressure by the pressure adjustment device, and supplied to the fuel injection valve.
The heretofore known apparatus shown in Patent Document 2 is configured of a motor-driven fuel pump (for example, a Wesco type fuel pump) in a fuel tank, a pressure adjustment device, a high pressure fuel pump driven by a cam attached to a cam shaft of an engine, a solenoid valve which controls the amount of fuel suctioned into the fuel pump, and a fuel injection valve, and by controlling a discharge amount of the high pressure fuel pump with respect to an injection amount of the fuel injection valve, a fuel pressure is optionally adjusted and fuel is supplied to the fuel injection valve.
The heretofore known apparatus shown in Patent Document 3 is arranged in such a way that two motorized fuel pumps are connected in series, and fuel is supplied to an engine by only the upstream side pump, and that a control fuel pressure is set variably by on/off controlling the downstream side pump.    Patent Document 1: JP-A-11-280584    Patent Document 2: JP-A-8-158971    Patent Document 3: JP-A-2007-255281
The heretofore known apparatus shown in Patent Document is one which realizes a low-cost fuel supply system, but as the fuel pressure is of a constant value, there is a limit to an atomization and flow range of fuel injected from the fuel injection valve.
Meanwhile, the heretofore known apparatus shown in Patent Document 2 is such that it carries out a control of the fuel injected from the fuel injection valve to an optimum particle size and injection amount by controlling the fuel pressure to a high pressure and a variable pressure, but as a fuel pressure sensor is needed in a fuel pipe, the high pressure fuel pump is fixed to the engine, and so on, there are many places to be changed on the engine side, leading to a high cost. Also, as a condition is such that the fuel pump is always driven even when an engine load fluctuates, the drive loss of the fuel pump is large, and there has been a problem in that a drive loss of the fuel pump when the engine load decreases, such as when idling, is large, or the like.
Furthermore, as the heretofore known apparatus shown in Patent Document 3 has a piping passage for supplying the fuel to the engine using only the upstream side pump, there has been a problem in that a piping configuration is complex, or the like.